1. Field of the Invention
The present invention relates to a device for welding together a first elongate work piece, such as a first rail, with a second elongate work piece, such as a second rail, end to end, by means of butt-weld resistance welding, particularly flash welding. The device comprises a structure and a carriage movably connected to the frame structure and slidable with respect thereto in the longitudinal direction of the work pieces, a first clamping device mounted on the frame structure or on parts associated therewith to fixedly clamp the first work piece, a second clamping device mounted on the carriage to fixedly clamp the second work piece, electrodes mounted on the frame structure and the carriage or on the clamping devices to heat the work pieces to the desired welding temperature, and a press element to press the facing ends of the heated work pieces against one another in the longitudinal direction of the work pieces to form a bonding welding joint.
2. Discussion of the Prior Art
One problem found in the use of a device of the above-mentioned kind for welding together for example a number of separate rails to form one long continuous rail is to ensure that each welding joint between two adjoining rails possesses a sufficiently high breaking strength and that each rail is securely welded in such a position that the upper part of the continuous rail thus formed, i.e. the so called rail head, is given a smooth running surface along its upper face and external flank, the so called track-forming edges, on which the railroad car wheels travel. To achieve this aim extremely careful relative orientation of the ends of two adjacent rails is required transversally as well as vertically. In EP-0 119 098 is described for instance an alignment apparatus comprising alignment means for alignment of two adjacent rail ends relatively to one another transversely as well as vertically. The alignment means are removed prior to the final welding together.
As the ends of the descrete rails usually are not manufactured to such a precision that their transverse end faces are completely straight and extend in parallel with each other when the work pieces are clamped in position in the frame and the carriage, it is common practice to subject them to a so called pre-flashing treatment prior to the welding-together proper in order to remove surplus material for reduction of the spacing between the ends. The rail flanks and/or the upper surfaces are not either manufactured to such a precision as to be completely straight, with the result that also when the work piece clamping members are positioned in parallel with one another a level running surface may not be obtained on the head of the finished continuous rail. The situation is further aggravated by the fact that in an area around the interconnecting welding joint the rail may be slightly deformed in the course of the subsequent cooling. Since the short-circuiting risks increase the larger the surface of contact between the transverse end faces, the risk of short-circuiting consequently becomes particularly imminent in the case of rails wherein the end faces are completely parallel.
Another problem found in several of the welding devices used today, such as flash welding machines, is the use of a press element in the form of one large pressure cylinder only, a so called upsetting cylinder, for the final pressing-together of the work pieces, such as rails, in which case molten and oxidised material is squeezed outwards from the facing ends of the work pieces while at the same time the material underneath which is heated to welding temperature, is welded together. Since in this kind of welding devices lack of space makes it difficult to position this pressure cylinder in such a manner that its longitudinal axis, i.e. centre line, coincides with the longitudinal axes of the work pieces, this kind of upsetting cylinder generates a moment of force which acts on the frame. The moment of force is of considerable magnitude, although the distance between the pressure cylinder longitudinal axis and said longitudinal axes is comparatively small, since the press force required to achieve the pressing together is of considerable magnitude. As a result, the welding device must be provided with a sufficiently large and heavy frame to absorb moments of force without buckling. From e.g. DE-1 068 402 it is already known to position upsetting cylinders symmetrically on either side of the rails to be welded together, in order to reduce the bending forces and torsional torque exerted on the frame during the upsetting operation. A condition for this effect to be obtained is to provide for excellent mechanical linear guidance, ensuring that the two work pieces are maintained in parallel relationship during the upsetting operation. These upsetting cylinders are controlled in common to provide a parallel movement of the upsetting cylinders. Should the relative heights or lateral extensions of the work pieces be changed during the upsetting operation, the moment of force of the upsetting cylinders will not be evened out and consequently the resulting moment of force must be absorbed by the frame, which therefore must be provided with an additional frame part. On the other hand, there is no mention in this patent publication that the welding device is configured in a manner allowing it to compensate for technical manufacturing deficiencies in the rails, such as curved track-forming edges.
A further problem found in prior art welding devices is that they do not offer any possibility of compensating for the kind of deformation of the welding joint formed during the welding operation that may be caused by the very pressing-together of the rail on account of the bending forces arising during this operation, which deformation may result in an unacceptable welding-together of the two work pieces, despite careful and accurate alignment of the work pieces prior to the welding-proper.